I have read at few sites that Pushyamitra Shunga (2nd century B.C.) had conducted two Ashwamedha yajnas. Did any king conduct Ashwamedha yagna after Pushyamitra ?
The last king to perform the Aśvamedha yajña was the 18th century Rajput ruler Sawai Jai Singh.
In History of Dharmaśāstra, Vol II Part II, P. V. Kane lists a few others:
In Indian Antiquary vol. VIII. p. 273 (at p. 278) we find that a general, called Udayacandra, of Nandivarma Pallavamalla (about the 9th century A. D.) defeated Pṛthivīvyāghra, king of Niṣadha, who had accompanied the horse in his horse sacrifice.
In Epigraphia Carnatica vol. X, Kolar No. 63, it is stated in an inscription of 757 A.D. that the Calukya emperor Pulakeśi was purified by his bath at an Aśvamedha.
In the very ancient Nanaghat Inscription (Archaeological Survey of Western India Reports vol. V. pp. 60-61) an Andhra king is described as having performed the Rājasūya, two Aśvamedhas, Gargatrirātra, Gavāmayana and Aṅgirasām-ayana.
In the first half of the 18th century Savai Jayasing, king of Amber, performed an Aśvamedha (vide Īśvaravilāsakāvya of Kṛṣṇa-kavi, D.C. Ms No. 273 of 1884-86 and Poona Orientalist, vol. II. pp.166-180).